The goal of this project is to improve the diagnosis and treatment of fetal disease. In the past, the Branch has pioneered the used of thin-gauge embryo-fetoscopy and endoscopic fetal surgery for the treatment of disorders of multiple gestation. This year the effort was focused on examining the accuracy of dating of pregnancy in the midtrimester. Gestational age assessment is an integral part of prenatal care. Inaccurate age assignment results in an increase in the rate of false positive and false negative cases, diagnostic errors, and unnecessary invasive procedures. The standard method of assigning duration of pregnancy is menstrual age and accuracy is dependent upon the reliability of menstrual history. However, as many as 45% of pregnant women had sub-optimal menstrual history. By far, the most popular alternative method of dating is fetal biometry with ultrasound. The Perinatology Research Branch conducted a study in collaboration with the Cornell University Medical Center to determine the accuracy of fetal biometry between 14 and 22 weeks in the prediction of gestational age in singleton, twin and triple gestations based upon population of patients who conceived as a result of in vitro fertilization. The results of this study demonstrated the size of the fetal head circumference was the most accurate predictor of gestational age of all parameters studied. The addition of one parameter (abdominal circumference or femur length) or two (abdominal circumference and femur length) significantly improved the accuracy of the prediction based on head circumference alone. The clinical practice in most obstetrical units is that discrepancy between gestational age by LMP disagrees by more than 14 days with that derived from fetal biometry in the second trimester, the LMP assessment is superseded by fetal biometry. Our study suggests that when a discrepancy of more than 7 days exists between the two, the biometric prediction should be given preference, provided there is no congenital anomaly or severe growth delay. A major finding of the study was that the dating formulae derived from singleton gestations can be used for dating multiple pregnancies. A simple average of the gestational age prediction of the two twins was found to be an accurate predictor of age. In the case of triplets, one day can be added to the average of the longest and shortest gestational age prediction among the triplets. Our observations imply that fetuses of twin gestations grow at a similar rate to singleton gestations during the midtrimester. However, data on triplets between 20 and 22 weeks is suggestive of a deceleration of growth, an issue which requires further investigation. We observed that the difference in the gestational age prediction among members of a multiple gestation averaged 2.2 days for twins (maximum = 7.2 days) and 4.2 days for triplets (maximum = 10.8 days). Therefore, when the difference among members of multiple gestation is two weeks or more, a search for fetal growth disorders, congenital anomalies, or a measurement error is justified.